Liver X-receptors (LXRs) are nuclear receptors that regulate the metabolism of several important lipids, including cholesterol and bile acids. Most of the cholesterol in plasma is transported on three major lipoprotein classes; VLDL cholesterol (VLDL-C), LDL cholesterol (LDL-C) and HDL cholesterol (HDL-C). Total cholesterol is the sum of all three lipoproteins. Both VLDL-C and LDL-C are associated with atherogenic processes while HDL-C is believed to facilitate cholesterol removal from tissues (e.g. atherosclerotic plaques) and thus have a protective effect on coronary heart disease.
LXR represents a novel intervention point to regulate the reverse cholesterol transport (RCT) pathway, i.e., the removal of cholesterol from peripheral tissues/cells and subsequent uptake via the liver for disposal. Removal of cellular cholesterol requires active transport of free cholesterol across the plasma membrane and onto HDL particles. This transfer of cholesterol from inside the cell and onto HDL in the plasma is mediated by ATP binding cassette 1 (ABCA1) transporter protein. The observation that LXR is a key transcriptional activator of ABCA1 in the macrophage, suggests that induction of LXR will lead to an increase in cholesterol efflux from the macrophage. In addition, it is known that LXR regulates the induction of other genes involved in RCT such as apoE and cholesterol ester transport protein (CETP), suggesting that activating the LXR pathway should also lead to increased uptake of cholesterol by the liver. Thus, activation of LXR by a small molecule ligand will lead to an up-regulation of ABCA1 and induction of the reverse cholesterol transport pathway thereby increasing cholesterol efflux to HDL-C and reducing the cholesterol content of atherosclerotic plaques.